Method for densifying nitrocellulose



United States This invention relates to the treatment of nitrocellulose and, more particularly, to the colloiding of nitrocellulose to produce fully colloided particles or objects thereof. In a specific aspect, the invention relates to the treatment of nitrocellulose having a nitrogen content from about 10.7% to about 11.2% by weight to change the physical form of such nitrocellulose from its customary fibrous form into solid, nonporous, fully colloided particles or objects thereof.

Nitrocellulose having a nitrogen content between about 10.7% and about 11.2% finds application in specialized nitrocellulose lacquers, adhesives and the like, in formulating nitrocellulose plastics, and in formulating black powder substitutes as well as other explosive and propellant compositions. Such nitrocellulose is produced and sold today in fibrous form. The bulk density of the product is low, causing relatively high shipping costs. This disadvantage is partially overcome by compressing the nitrocellulose with a ram to increase barrel loadings. As a result of compression in packing, nitrocellulose users find the fibrous material quite difficult to unload from the barrels. An additional disadvantage of conventional commercial fibrous nitrocellulose is the tendency to agglomerate and form large, slow-dissolving lumps when added to solvents in certain types of dissolving equipment.

Therefore, the primary object of this invention is to produce an improved form of nitrocellulose which can be transported more economically, unloaded from barrels more easily, and dissolved more rapidly in solvents.

A further object of the invention is to provide a new and improved method for converting fibrous nitrocellulose having a nitrogen content from about 10.7% to about 11.2% into solid, nonporous, fully colloided particles or objects thereof.

Other objects will become apparent from the following description of the invention, the novel features and combinations being set forth in the appended claims.

Generally described, these objectives and others are accomplished in accordance with this invention by dissolving nitrocellulose having from about 10.7% to about 11.2% nitrogen content by weight in glycol having the formula 1 wherein R is an aliphatic hydrocarbon radical having from 2 to 6 carbon atoms, preforming the resulting solution, introducing the preformed solution into a water bath containing not more than about 90% by weight of said glycol at the point of original contact of said solution with said water bath to gel the nitrocellulose, leaching the gelled nitrocellulose in the presence of water until a predetermined amount of the glycol originally present in the gelled nitrocellulose has been replaced by water, and subjecting the resulting leached nitrocellulose to drying conditions to remove water and produce colloided nitro cellulose.

In the above-described process, the surface of the body of preformed solution of nitrocellulose in glycol upon contact with the water bath is gelled at the instant of contact and a tough envelope of gelled nitrocellulose *atent composition is formed about the exterior of the body. For convenience, this phenomenon is termed shockgelling. This envelope of shock-gelled nitrocellulose composition is of sufficient toughness that the body retains its preformed shape, and is also permeable to the passage of water into the body and the passage of glycol out of the body. Accordingly, as the shock-gelled body is retained in the water bath or is otherwise further contacted with the aqueous medium, water leaches the glycol from the shock-gelled body and water replaces the glycol in the body of composition until an equilibrium condition is approached at which the concentration of glycol is substantially the same in the particle or body as in the surrounding leaching bath. Obviously, of course, gelation progresses inwardly within the shock-gelled particle or body as water permeates thereinto until the entire particle or body is in a gelled state. Thus, a solution of nitrocellulose in glycol will be gelled when introduced into the water bath, and the amount of glycol removed from the gelled composition by leaching in the water bath will depend upon the concentration of glycol in the water bath and the time of contact. If the leaching process is conducted stepwise and the gelled material is successively introduced into fresh leaching baths, thepoint will be reached at which substantially all 2f the glycol will be removed from the gelled material. Generally, however, it is more convenient and economical to leach the glycol from the gelled material by continuous countercurrent extraction.

It has been found' that very little shrinkage of the gelled material occurs during the leaching process, even when water has replaced substantially all of the glycol in the gelled material. Consequently, the gelled material at conclusion of the leaching process is still a swollen, highly attenuated gel, somewhat rubbery and elastic in behavior. For convenience, the gelled material upon completion of leaching, but prior to drying, is termed water-logged. However, upon subjecting to drying conditions, the waterlogged material shrinks uniformly as the water is removed in the drying operation to final dimensions determined by the original concentration of nitrocellulose in the glycol solution. In the final dry condition the nitrocellulose material is in the form of hard, tough, nonporous, fully colloided particles or objects which retain substantially the physical shape of the shock-gelled bodies of material subjected to leaching.

Having generally described the invention, the following examples set forth some specific embodiments of the in-- vention. It is to be understood, however, that these examples, while illustrative, are not to be construed as a limitation of the invention.

Example 1 Five parts by weight of dry fibrous nitrocellulose having a :loose bulk density of about 12 lb./cu. ft. and a barrel packed density of about 23 lb./cu. ft., a nitrogen content of 11.0% by weight and a standard ASTM falling ball viscosity characteristic of 13 seconds were dissolved in parts by weight of propylene glycol (1,2-propanediol).

This solution was poured into a metal container, the bottom of which contained perforations about 0.09 inch in diameter. The droplets of solution emitted from these holes were allowed to fall about 18 inches through air before striking the surface of a Water bath. During this fall the droplets became essentially spherical. The surface of the droplets gelled instantly upon contact with the water bath and were sufliciently tough to withstand passage through the water surface without substantial deformation. The shock-gelled balls thus produced were leached in running water for a period of 2 hours, at which time they were about 0.15 inch in diameter. The waterlogged balls were removed from the water and dried over- Example 2 Five parts by weight of the same fibrous nitrocellulose employed in Example 1 were dissolved in 95 parts by weight of 1,5- pentanediol, and the resulting solution was shock-gelled, leached, and the water-logged particles were dried substantially as set forth in Example 1. The resulting densified nitrocellulose product was substantially the same as the product prepared in Example 1.

Examples 3-5 Dry fibrous nitrocellulose, having a nitrogen content of 10.9% by weight and a. standard ASTM falling ball viscosity characteristic of about 13.5 seconds, was employed to prepare the following solutions in various glycols.

Weigh t of Percent Nitrocellu- Ex. No. Solvent by lose Dis- Weight solved,

Percent 1,2-Propanediol. 95.3 4. 7 1,3-Propanediol. 95 5.0 1,5-Pentanecliol 99. 0 1.0

The above solutions were shock-gelled, leached, and the water-logged particles were dried substantially as set forth in Example 1. The resulting dried nitrocellulose in all cases was a free-flowing, fast-dissolving and high bulk density product in the form of solid, nonporous, fully-colloided, tiny spheroids.

Examples 6-13 Dry fibrous nitrocellulose, having a nitrogen content of 11.0% by weight and a standard ASTM falling ball viscosity characteristic of A second, was employed to prepare the following solutions in various glycols.

Weight of Percent Nitrocellu- Ex. No. Solvent by lose Dis- Weight solved,

Percent Ethylene glycol. 95. 3 4. 7 1,2-Propanediol. 85.0 15. 0 l,3-Propanediol 95.0 5.0 1,3-Butanediol 98. 6 1. 4 1,4-Butanediol 98.5 1. 5 1,5-Pentanediol 90. 0 10.0 2-Methyl-2,4-pcntane 01 95.0 5.0 2,5-Hexancdiol 99. 7 0. 3

The above solutions were shock-gelled, leached and the water-logged particles were dried substantially as set forth in Example 1. The resulting dried nitrocellulose in all cases was a free-flowing, fast-dissolving, and high bulk density product in the form of solid, nonporous, fully colloided, tiny spheroids.

Examples 14-16 Dry fibrous nitrocellulose, having a nitrogen content of 11.06% by weight and a standard ASTM falling ball viscosity characteristic of 5 seconds, was employed to prepare the following solutions in various glycols.

Weight of Percent N itro cellu- Ex. No. Solvent by lose Dis- Weight solved,

Percent 1,2-Propanediol. 95. 3 4. 7 1,3-Propanediol. 95.0 5.0 1,5 Pentanediol 99. 0 1.0

The above solutions Were shock-gelled, leached and the water-logged particles were dried substantially as set forth in Example 1. The resulting dried nitrocellulose in all cases was a free-flowing, fast-dissolving, and high bulk density product in the form of solid, nonporous, fully colloided, tiny spheroids.

Examples 17-22 Dry fibrous nitrocellulose, having a nitrogen content of 11.1% by weight and a standard ASTM falling ball viscosity characteristic of 13 seconds, was employed to prepare the following solutions in various glycols.

Weight of Percent Nitrocellu- Ex. No. Solvent y lose Dis- Weight solved,

Percent Ethylene glycol 98. 3 1. 7 91.0 9.0 95. 0 5. 0 90. 8 0.2 1,5-Pentanedi0l 95.3 4. 7 2-Methyl-2,4-pentanediol. 95. 0 5.0

The above solutions were shock-gelled, leached and the water-logged particles were dried substantially as set forth in Example 1. The resulting dried nitrocellulose in all cases was a free-flowing, fast-dissolving and high bulk density product in the form of solid, nonporous, fully colloided, tiny spheroids.

Examples 23-25 Dry fibrous nitrocellulose, having a nitrogen content of 11.0% by weight and a standard ASTM falling ball viscosity characteristic of seconds, was employed to prepare the following solutions in various glycols.

Weight of Percent N i tro cell u- Ex. No. Solvent y lose Dis- Wcight solved,

Percent 1,2-Propanediol 95. 3 4. 7 1,3-Propanediol 95.0 5. 0 1,5-Pentanediol 95. 0 5.0

The above solutions were shock-gelled, leached and the water-logged particles were dried substantially as set forth in Example 1. The resulting dried nitrocellulose in all cases was a free-flowing, fast-dissolving and high bulk density product in the form of solid, nonporous, fully colloided, tiny spheroids.

Example 26 Five parts by weight of dry fibrous nitrocellulose, having a nitrogen content of 11.2% by weight and a standard ASTM falling ball viscosity characteristic of 5 seconds, was dissolved in parts by weight of 1,2-propanediol, and the resulting solution was shock-gelled, leached, and the water-logged particles were dried substantially as set forth in Example 1. The resulting densified nitrocellulose product was similar in physical form and properties to the product produced in Example 1.

Example 27 Five parts by weight of dry fibrous nitrocellulose, having a nitrogen content of 10.7% by weight and a standard ASTM falling ball viscosity characteristic of 4 seconds, was dissolved in 95 parts by weight of 2-butene-l,4 diol, and the resulting solution was shock-gelled, leached, and the water-logged particles were dried substantially as set forth in Example 1. The resulting densified nitrocellulose product was similar in physical form and properties to the product produced in Example 1.

Example 28 Ten parts by weight of dry fibrous nitrocellulose, having a nitrogen content of 11.1% by weight and a standard ASTM falling ball viscosity characteristic of 5 seconds, and 0.1 part by weight of 2-nitrodiphenylainine were dissolved in 89.9 parts by Weight of 1,2-propylene glycol. The resulting solution was extruded through a 0.19 inch diameter opening using a pressure of 3 p.s.i. into a water bath to form a continuous coil of shock-gelled nitrocellulose about 0.23 inch in diameter, which was cut below the water level in the bath into strands about 6 inches in length. The strands were leached and the water-logged strands were dried substantially as set forth in Example 1. The resulting nitrocellulose product was a hard, high density, fully colloided material, each strand being about 0.075 inch in diameter and 3 inches in length.

This invention is specific both with respect to the nitrogen content of the nitrocellulose employed and with respect to the glycols employed to prepare the nitrocellulose solutions.

Nitrocelluloses in accordance with this invention have nitrogen contents between about 10.7% and about 11.2% by weight. Nitrooelluloses having nitrogen contents of about 10.6% or lower and about 11.3% or higher by weight are insoluble in the glycols of this invention and are, therefore, inoperative in accordance with this invention. It is to be understood, however, that the above limitation of about 10.7% and about 11.2% nitrogen content permits leeway of a few hundredths of a percent on both the low side and the high side of the defined operative range. There is no limitation with respect to the viscosity characteristic of the nitrocellulose employed, and all commercially available viscosity types of nitrocellulose, ranging in viscosities from cps. to 377,000 cps. (1000-second type) in 12.2% solution in a solvent composed of 20% ethyl acetate, 25% ethyl alcohol, and 55% benzene, are densified in accordance with this invention.

All glycols having the formula /OH R wherein R is an aliphatic hydrocarbon radical having from 2 to 6 carbon atoms are operativefor the purposes of this invention. This includes all straight-chain, branched-chain and unsaturated aliphatic hydrocarbon radicals having-2 to 6-carbon atoms. Glycols wherein R is an aliphatic hydrocarbon radical having 7 or more carbon atoms are such poor solvents for the nitrocelluloses of this invention that they dissolve only vanishingly small amounts of nitrocellulose and are, therefore, not practical to employ. Typical glycols within the scope of this inethylene glycol or dipropylene glycol, and the like, are not operative for the purposes of this invention, for solutions of nitrocellulose having about 10.7% to about 11.2% nitrogen in such glycols do not shock-gel in accordance with this invention. Such solutions upon contact with water either do not gel at all or if they do gel, there is formed a fibrous or porous precipitate.

In practicing this invention, it has been found generally that glycols having an odd number of carbon atoms in the hydrocarbon radical are more active solvents for the nitrocelluloses of this invention than glycols having an even number of carbon atoms in the hydrocarbon radical. It has also been observed that introduction of an unsaturation into the molecule greatly enhances nitrocellulose solubility, for example, 2-butene-1,4-diol is a much better solvent than 1,4-butanediol. Similarly, a branchedchain glycol usually has greater solvent power for the nitrocelluloses of this invention than straight-chain glycols of the same number of carbon atoms. Generally, the more active a glycol solvent is for the nitrocelluloses of this invention, the greater is the amount of any given nitrocellulose which can be dissolved therein to produce a solution of any preselected viscosity.

Another factor which has been observed in preparing glycol solutions of the nitrocelluloses of this invention is that such glycol solutions exhibit the phenomenon of thermal gelling to a greater or lesser extent, depending upon the glycol being employed. By thermal gelling is meant that a temperature will be found for any given glycol solution of nitrocellulose, above which the solution will gel. Generally, glycols wherein R is a saturated straight-chain aliphatic hydrocarbon radical having an even number of carbon atoms, such as ethylene glycol, the straight-chain butanediols and the straight'chain hexanediols exhibit this thermal gelling phenomenon to a more pronounced degree than glycols in which R is a branched chain, a straight chain of odd number of carbon atoms, or an unsaturated aliphatic hydrocarbon radical. Because of this thermal gelling phenomenon, it has been found necessary to cool ethylene glycol and other straight-chain glycols to about 0 C. in order to effect solution of the nitrocellulose of this invention therein. In fact, cooling any of the glycols of this invention improves their solvent power for the nitrocelluloses of this invention.

The solution of nitrocellulose in glycol may be'preformed for introduction into the gelling bath in a number of ways, depending largely upon the size and shape of the particles or objects desired. When very small particles of nitrocellulose composition on the order of microns or less in size are desired, it is most convenient to atomize the solution from a suitable spray nozzle or orifice to forma spray of minute droplets, and the droplets thus formed are allowed to fall into the gelling bath. If desired, the spray droplets may first be allowed to fall by atomizing from a spray nozzle or orifice, the viscosity of the solution obviously must be adjusted to a level which;

can be suitably atomized by spraying, as is well recognized by those skilled in the art.

It will be apparent, of course, that the viscosity of the glycol solution of nitrocellulose will depend both upon the viscosity characteristic of the nitrocellulose employed as well as-the amount of nitrocellulose which is dissolved in the glycol. It follows-therefore, that for any desired solution viscosity, the amount of nitrocellulose necessary to produce that desired viscosity will decrease as the viscosity characteristic of the nitrocellulose increases. It also follows that with any selected nitrocellulose, the viscosity of the'solution will increase with increasing amount of nitrocellulose dissolved therein. The art is fully cognizant of of nitrocellulose in glycol is formed into droplets of the :techniques employed :1

process for manufacture of smokeless powder. By this technique the stiff paste-like solution of nitrocellulose is preformed by pressure extrusion through a die of any desired cross-sectional area and configuration, and the extruded shape is introduced into the water bath for gelation and removal of the glycol by leaching. In this technique the pressure-extruded shape may be cut into predetermined lengths prior to introduction into the gelling bath, after gelation, or after leaching, as convenient or desired.

It has also been found that the glycol solutions of nitrocellulose in accordance with this invention can be cast into water-wet molds to shock-gel the castings, after which the surface-gelled castings are leached in a water bath, and the water-logged castings are then dried. Similarly, objects can be coated with the glycol solutions of this invention and such coatings can then be shock-gelled, leached and dried to produce tough, nonporous, fully colloided nitrocellulose coatings thereon.

In all of these techniques the gelled particle, shape or coating shrinks uniformly as the water is removed in the drying operation to final dimensions determined by the original concentration of nitrocellulose in the glycol solution which is shock-gelled and then leached.

From the foregoing description, itwill be seen that the concentration of nitrocellulose, in glycol solution is largely a matter of choice and convenience, depending somewhat, of course, on the method of preforming which is employed, and the size and shape of particle or object desired. A controlling factor at the low end of concentration will be the physical strength of the shock-gelled material. In general, concentrations much under 2% will be found to be somewhat tender to handle properly for processing. The upper limit of concentration is detern' the celluloid a rt or'i'n the solvent 'thepamcles y n of met'aflie'ticai reac FsecQnd and, I es of thf higher vise tio'ris' or' by simple evaporation of solution-impregnated material.

Alter leaching, the water-logged nitrocellulose may be exchanged with other fluids such as nonsolvent alcohols or other water-soluble nonsolvents for nitrocellulose, and by a suitable succession of impregnations the nitrocellulose system can ultimately be made to contain high percentages of hydrocarbons or the like. For example, the Water present in the water-logged nitrocellulose may be displaced by exhaustive extraction with isopropyl alcohol, and the alcohol in turn may be displaced by a toluene solution of mineral oil. Upon final drying of the nitrocellulose structure, a tough, rugged, elastic, fully colloided nitrocellulose plastic composition plasticized with mineral oil is obtained which is characterized by surprisingly high strength and impact resistance.

The water-logged nitrocellulose compositions of this invention may be partially dried to a Water content of 20% to 30% Water for storage and shipment if desired. Alternatively, the water-logged material may be dehydrated for storage and shipment by displacement of the water with a nonsolvent volatile alcohol such as isopropyl alcohol and drying to a volatile content of about 25% to 35% by weight. If desired, the water-logged material may be partly dried to a water content of about 30% by weight and then the residual Water may be displaced with isopropanol, or the like.

It is apparent from the foregoing description that this invention provdes a novel and improved process for converting fibrous nitrocelulose having a nitrogen content from about 10.7% to about 11.2% into solid, nonporous, fully colloided particles or objects thereof. The improved form of nitrocellulose produced can be transported more economically, unloaded from barrels more easily, and dissolves more rapidly in solvents than commercial fibrous nitrocellulose since there is no tendency for the densified nitrocellulose of this invention to agglomerate into large, slow-dissolving lumps. The higher bulk density of the nitrocellulose products of this invention results in transportation savngs. Commercial fibrous nitrocellulose of 10.7% to 11.2% nitrogen content has a loose bulk density of about 12 1b. (dry basis) per cubic foot and is rammed into barrels to a bulk density of about 23 lb./cu. ft. (dry basis). Fully colloided nitrocellulose in accordance with this invention in the form of tiny, solid, nonporous spheroids averaging 0.05 inch diameter has a bulk density on the order of 60 to 65 lb./ cu. ft. (dry basis), thus permitting substantially heavier barrel loadings than with fibrous nitrocellulose. Moreover, the fully colloided nitrocellulose particles of this invention can be poured from the barrels for very easy unloading. Present commercial fibrous nitrocellulose must be dug out of the barrel. The elimination of the costly and labor-consuming pressing or centrifuging and blockbreaking operations usually following nitrocellulose dehydration is an important advantage of the fully colloided nitrocellulose particles of this nvention. Furthermore, the free-flowing, fast-dissolving and high bulk density nitrocellulose products of this invention can be used in any application where commercial fibrous nitrocellulose of the same nitrogen content is now used, such as lacquers, adhesives, plastics, propellants, and the like.

What I claim and desire to protect by letters Patent 1. A process for colloiding nitrocellulose which comprises dissolving nitrocellulose having from about 10.7% to about 11.2% nitrogen content by weight in a solvent consisting of Water soluble glycol having the formula wherein R is an aliphatic hydrocarbon radical having from 2 to 6 carbon atoms, preforming the resulting nitrocellulose solution into shaped bodies, introducing the shaped bodies of preformed nitrocellulose solution into a water bath containing not more than about 90% by weight of said glycol at the point of original contact of said shaped bodies with said water bath to shock-gel said shaped bodies by rapidly forming a tough envelope of gelled nitrocellulose on the surfaces of said bodies so that said bodies retain their preformed shape, leaching the resulting shockgelled bodies in the presence of additional water until substantially all of the glycol present therein has been replaced by water to form water-logged gels, and subjecting the resulting water-logged gels to drying conditions to remove water and produce dry, nonporous, fully colloided nitrocellulose bodies having substantially the same physical shape as the shock gelled bodies subjected to leaching.

2. A process according to claim 1 in which the solution is poured through a perforated plate to form droplets and the droplets thus formed are allowed to fall into the bath.

3. A process according to claim 1 in which the solution is gravity extruded into the bath.

4. A process according to claim 1 in which the solution is preformed by pressing through an orifice, and the preformed solution is introduced into the bath.

5. A process according to claim 1 in which which the solution is atomized by spraying, and the atomized spray particles are introduced into the bath.

6. A process according to claim 1 in which said glycol having the formula is selected from the group of glycols in which R consists of 4-carbon, S-carbon and 6-carbon unsaturated aliphatic hydrocarbon radicals.

7. A process according to claim 1 in which said glycol having the formula is selected from the group of glycols in which R consists of 3-carbon and S-carbon saturated aliphatic hydrocarbon radicals.

8. A process according to claim 1 in which the glycol is 1,2-propanediol.

9. A process according to claim 1 in which the glycol is 1,3-propanediol.

10. A process according to claim 1 in which the glycol is 2-butene-1,4-diol.

References Cited in the file of this patent UNITED STATES PATENTS 1,640,712 Moran Aug. 30, 1927 1,746,543 Lowry Feb. 11, 1930 2,021,837 Davidson Nov. 19, 1935 2,230,100 Aaron et a1. Ian. 28, 1941 

1. A PROCESS FOR COLLOIDING NITROCELLULOSE WHICH COMPRISES DISSOLVING NITROCELLULOSE HAVINGFROM ABOUT 10.7% TO ABOUT 11.2% NITROGEN CONTENT BY WEIGHT IN A SOLVENT CONSISTING OF WATER SOLUBLE GLYCOL HAVING THE FORMULA 